Programming Forums > Application Development > Java

chillypacman · May 30th, 2006, 10:09 AM

I can't seem to do it, I'm assuming its just like creating an array of anything else right?

So if I have one class:
[code]
import java.util.*;

public class abcd
{
public static void main(String [] args)
{
abc[] link = new abc[5];

link[0].number();
}
}

and the other one its supposed to create an array of:

(Toggle Plain Text)

import java.util.*;

public class abc
{
	Scanner keyboard = new Scanner(System.in);
	private int num;
	
	public void number()
	{
		num = keyboard.nextInt();
	}
	
	public int cda(int num)
	{	
		this.num = num;
		return num;										
	}
}

why do I get a null pointer exception at runtime?

It seems pretty straightforward to me...

titaniumdecoy · May 30th, 2006, 10:53 AM

You have to initialize each index of the array separately.

NSchnarr · May 30th, 2006, 10:59 AM

WHat you have done is created an array which is ready to store abc class objects. However, you need to assign objects to it before you can try calling functions (initialize each index as titaniumdecoy said). Storing objects in arrays does work the same as other things, strings for example. You would need to assign a string to each index before you called string methods, its the same idea.

chillypacman · May 30th, 2006, 11:06 AM

lol, yeah I see now... I was on it for like an hour, I kept on going over and over it, I really didn't see anything wrong with it, except the most obvious thing...

but what if I initialise values within the class?

for instance if num = 1, wouldn't that mean its already initialised?

Just wondering...

oh and how do I intialize values for the array?

NSchnarr · May 30th, 2006, 11:30 AM

You havent created an abc class object yet, so you would be unable to set num = 1, lol.
This is what you need to do.

(Toggle Plain Text)

abc obj1 = new abc();
abc obj2 = new abc();
link[0] = obj1;
link[1] = obj2;

this will at least store them for you, now as for working with then you will have to play around.

chillypacman · May 30th, 2006, 11:43 AM

ok, I see.

but it does look like it will take a while, I mean I have to create a different object for every index in the array then I have to store them each in the array.

Couldn't I just skip the first part? I mean, isn't an array basically a collection of similar objects?

NSchnarr · May 30th, 2006, 1:13 PM

no, an array is a place to store similar objects, you have to create them first.

azurik · May 31st, 2006, 6:39 AM

Quote:

Originally Posted by chillypacman

ok, I see.

but it does look like it will take a while, I mean I have to create a different object for every index in the array then I have to store them each in the array.

Couldn't I just skip the first part? I mean, isn't an array basically a collection of similar objects?

What you could do is:

(Toggle Plain Text)

final int ARRAY_LENGTH = 10;
abc[] array = new abc[ARRAY_LENGTH];
for (int arrayPos = 0; arrayPos < ARRAY_LENGTH; arrayPos++)
      array[arrayPos] = new abc();

That code will fill the array with initalized abc objects. If you ever need to call a method in the 4th array position, for instance, you would then do:

array[4-1].someMethod();

I'm not positive if it's conventional to do it that way, but I do it all the time anyway. :p

May 30th, 2006, 10:09 AM	#1
chillypacman Newbie Join Date: Mar 2006 Posts: 21 Rep Power: 0	creating an array of an object/class? I can't seem to do it, I'm assuming its just like creating an array of anything else right? So if I have one class: [code] import java.util.; public class abcd { public static void main(String [] args) { abc[] link = new abc[5]; link[0].number(); } } and the other one its supposed to create an array of: (Toggle Plain Text) import java.util.; public class abc { Scanner keyboard = new Scanner(System.in); private int num; public void number() { num = keyboard.nextInt(); } public int cda(int num) { this.num = num; return num; } } import java.util.*; public class abc { Scanner keyboard = new Scanner(System.in); private int num; public void number() { num = keyboard.nextInt(); } public int cda(int num) { this.num = num; return num; } } why do I get a null pointer exception at runtime? It seems pretty straightforward to me...

May 30th, 2006, 11:30 AM	#5
NSchnarr Newbie Join Date: May 2006 Posts: 28 Rep Power: 0	You havent created an abc class object yet, so you would be unable to set num = 1, lol. This is what you need to do. (Toggle Plain Text) abc obj1 = new abc(); abc obj2 = new abc(); link[0] = obj1; link[1] = obj2; abc obj1 = new abc(); abc obj2 = new abc(); link[0] = obj1; link[1] = obj2; this will at least store them for you, now as for working with then you will have to play around.

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May 30th, 2006, 10:53 AM	#2
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 904 Rep Power: 3	You have to initialize each index of the array separately.

May 30th, 2006, 10:59 AM	#3
NSchnarr Newbie Join Date: May 2006 Posts: 28 Rep Power: 0	WHat you have done is created an array which is ready to store abc class objects. However, you need to assign objects to it before you can try calling functions (initialize each index as titaniumdecoy said). Storing objects in arrays does work the same as other things, strings for example. You would need to assign a string to each index before you called string methods, its the same idea.

May 30th, 2006, 11:06 AM	#4
chillypacman Newbie Join Date: Mar 2006 Posts: 21 Rep Power: 0	lol, yeah I see now... I was on it for like an hour, I kept on going over and over it, I really didn't see anything wrong with it, except the most obvious thing... but what if I initialise values within the class? for instance if num = 1, wouldn't that mean its already initialised? Just wondering... oh and how do I intialize values for the array?

May 30th, 2006, 11:43 AM	#6
chillypacman Newbie Join Date: Mar 2006 Posts: 21 Rep Power: 0	ok, I see. but it does look like it will take a while, I mean I have to create a different object for every index in the array then I have to store them each in the array. Couldn't I just skip the first part? I mean, isn't an array basically a collection of similar objects?

May 30th, 2006, 1:13 PM	#7
NSchnarr Newbie Join Date: May 2006 Posts: 28 Rep Power: 0	no, an array is a place to store similar objects, you have to create them first.

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Programming Forums > Application Development > Java

creating an array of an object/class?